In recent years, a magnetic shift register termed “racetrack memory” has come into the spotlight as a dense memory technology. Racetrack memory operates by pushing magnetic domain walls (i.e., the “walls” that define regions of opposing magnetization in a magnetic material) along a track composed of a magnetic wire. In some prior art devices, stable domain wall positions are created by adding “notches” along a track. In racetrack memories, the domain wall shifting mechanism is believed to be spin-transfer torque (STT), where a spin-polarized input current imparts a torque on local magnetic moments, causing them to align in the direction of the electron spin. FIG. 1 (adapted from FIG. 4A of U.S. Pat. No. 7,551,469, incorporated by reference herein) shows an exemplary, prior art 2D racetrack memory. The bits of this racetrack memory are read out serially via a magnetic sensor, such as a magnetic tunnel junction (MTJ) or giant magnetoresistive (GMR) stack, positioned somewhere along the chain. 3D versions of such structures have been proposed, but the fabrication process is prohibitively complex.
While racetrack memory remains a potentially interesting technology that might have commercial value in the future, there are numerous problems that impede its commercial deployment today. Racetrack memories are complicated to control, requiring precise timing and pulse generation to move the domain walls along the track simultaneously and at the appropriate speed, as well as having them stop in the low energy positions. Most importantly, however, adjacent domain walls in the track—effectively the bits of data—occasionally annihilate each other during motion. This annihilation of domain walls represents a loss of data in the memory.
A principal objective of the present invention is to enable a nonvolatile alternative to DRAM or Flash. As discussed in greater detail below, this objective is achieved, at least in part, through use of a new “magnetic shift register” that avoids the bit annihilation problem that plagues racetrack memories.